This invention relates generally to magnetic resonance imaging (MRI), and more particularly, the invention relates to field map estimation from multi-echo MRI sequences.
Magnetic resonance imaging (MRI) is a non-invasive method for the analysis of materials and is an approach to medical imaging. It does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During an MRI experiment, each nuclear spin responds to four different effects: precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MRI experiments, a combination of these effects occurs periodically.
Reliable and uniform fat suppression in MRI is clinically important, as fat signals appear bright in many fast imaging sequences, such as fast spin echo, spoiled gradient echo, and balanced SSFP. Unless suppressed, bright fat signals may obscure underlying pathology, degrading the diagnostic value of MR images. Common fat suppression techniques include fat saturation, spectral-spatial water selective excitation, STIR (short-tau inversion recovery), and water-fat separation methods based on chemical-shift induced phase differences. The latter group of methods, derived from the two-point Dixon method, is less susceptible to magnetic field (B0) inhomogeneities, since field inhomogeneities can be corrected before separation. The potential to correct for field inhomogeneities has led to many variations of multi-point water-fat separation methods.